Remove superfluous checks for CONFIG_BLK_DEV_INITRD from initramfs.c

Given that init/Makefile includes initramfs.c in the build only if
CONFIG_BLK_DEV_INITRD is defined, there seems to be no point checking for
it yet again.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>